1. Field of the Invention
This invention relates to a carrier for optical semiconductor device and a mounting structure thereof used in optical fiber communication system or optical local area network, and further relates to an optical semiconductor module on which optical wave guide elements such as the carrier for optical semiconductor device, an optical fiber coupled thereto and an optical wave guide channel are mounted.
2. Description of the Related Art
In recent years, optical fiber communication is realized in fields of cable television (CATV) and public data network. Furthermore, an optical module having high speed and high accuracy is realized by a module structure called xe2x80x9ccoaxial typexe2x80x9d or xe2x80x9cdual-inline typexe2x80x9d. These elements are practiced mainly in a field called xe2x80x9ctrunk linexe2x80x9d.
On the other hand, another optical module is developed by using a technology for mounting an optical semiconductor device and an optical fiber on a silicon substrate with high positioning accuracy owing to mechanical precision. This optical module is aimed to be practiced mainly in a field called xe2x80x9csubscriber systemxe2x80x9d, so that it is required to be downsized, to be thinner and to be inexpensive.
Examples of conventional mounting structure of a surface sensing type optical semiconductor device (photodiode) are described.
FIG. 26 shows an example of a carrier (mounting base) 40 having a cubic shape. A photodiode will be mounted on the carrier 40. Electrodes 411 and 412, to which anode and cathode of the photodiode are respectively connected, are formed on adjoining first face 41 and second face 42 of the carrier 40. Portions of the electrodes 411 and 412 on the first face 41 are electrically connected to the other portions on the second face 42 at the boundary of the first face 41 and the second face 42.
FIG. 27 shows that the optical semiconductor device such as a PIN type photodiode 20 is mounted on the carrier 40. A size of the photodiode 20 is, for example, a square of about 500 xcexcm and a thickness of about 200 xcexcm. A photo sensing area of the photodiode 20 is a circular having a diameter about 200 xcexcm. An electrode 21 formed on a photo sensing plane is electrically connected with the electrode 412 by a bonding wire 31. Another electrode 22 formed on a rear face is directly connected with the electrode 411 by a connecting element such as a solder of AuSn.
FIGS. 28A to 28C show a mounting structure of the carrier 40 with the photodiode 20 on a silicon substrate 30. The photodiode 20 is held in a manner so that the photo sensing plane is vertical to a principal plane of the silicon substrate 30 via the carrier 40. By such a configuration, an optical fiber (not shown in the figure) mounted in parallel with the principal plane on the silicon substrate 30 is optically coupled with the photodiode 20. The portions of the electrode 411 and 412 on the first face 41 are respectively connected to electrodes (not shown) formed on the silicon substrate 30 by bonding wires 32 and 33, so that electric power can be supplied to the photodiode 20. The carrier 40 is generally formed by a ceramic material such as alumina. The electrodes 411 and 412 on the carrier 40 are formed on the first face 41 and the second face 42 of the carrier 40 by printing method using a paste including a filler.
In a process for forming the electrodes 411 and 412 on the first face 41 and the second face 42 of the carrier 40, the portions of the electrodes 411 and 412 on the first face 41 and the other portions of them on the second face 42 cannot be formed at the same time. At first, the carrier 40 is disposed in a manner so that the first face 41 be disposed at the top end. The portions of the electrodes 411 and 412 on the first face 41 are formed. After that, the carrier 40 is picked up and turned by a handle so that the second face 42 be disposed at the top. Subsequently, the other portions of the electrodes 411 and 412 on the second face 42 are formed. Thus, the process for forming the electrode 411 and 412 on the first face 41 and the second face 42 of the carrier 40 is complex, and the productivity thereof is much lower.
Furthermore, accuracy of the relative position of the portions of the electrode 411 and 412 on the first face 41 and the other portions of them on the second face 42 depends on the positioning accuracy of the carrier 40 by the handle, so that the gap between the electrodes 411 and 412 and the width of them cannot be made so narrower, for example, less than about 70 xcexcm. This limitation of the width of the electrodes 411 and 412 disturbs to decrease the impedance of the electrodes 411 and 412, to improve the high frequency characteristics of the optical semiconductor device and to downsize the optical semiconductor module.
Furthermore, the smaller the size of the carrier becomes, the more difficult to handle the carrier, so that the productivity becomes much lower. Thus, it is substantially impossible to downsize the carrier smaller than the cubic having each side length of 2 mm by the conventional method.
In other words, the size and the cost of the carrier for optical semiconductor device are in a relation of trade off, so that the cost of the carrier becomes very expensive by downsizing and high accuracy of the carrier, and there are physical limitations in the downsizing and high accuracy.
Another mounting structure of the photodiode directly mounted on the silicon substrate without using the carrier is shown in Publication Gazette of Japanese Patent Application Hei 8-94887. A slant face, on which the photodiode is fixed, is formed on the silicon substrate at a position on production of mounting groove for the optical fiber. When the optical fiber and the photodiode are mounted on the silicon substrate, the exit plane of the optical fiber faces the photo sensing plane of the photodiode but they are not parallel. The electrode on the rear face of the photodiode directly contacts the electrode formed on the slant face, and the electrode on the photo sensing plane is connected to the electrode by the bonding wire.
Since the electrode of the photodiode is slanted with respect to the principal plane, the wiring process for connecting the electrode of the photodiode and the electrode on the silicon substrate becomes very difficult. Furthermore, the angle of the slant surface with respect to the principal plane of the silicon substrate is restricted by workability for forming and/or for wiring the electrode on the slant face, so that tolerances of photosensitivity and positioning of the photodiode become smaller than those when the photo sensing plane of the photodiode is disposed perpendicular to the optical path of the light beam emitted from the optical fiber.
Still another method for mounting the photodiode directly on the silicon substrate is shown in Publication Gazette of Japanese Patent Application Hei 9-54228. A total reflection mirror having a reflection angle about 45 degrees with respect to the exit plane of the optical fiber is formed on an end of the mounting groove for the optical fiber on the silicon substrate. The photodiode is directly mounted on the silicon substrate in a manner so that a part of the photo sensing plane overhangs for facing the mirror. An optical path of a light beam emitted from the exit plane of the optical fiber is bent about 90 degrees toward the photo sensing plane of the photodiode by the mirror.
Since the total reflection mirror is formed on an end of the mounting groove, the shape and the manufacturing process of the mounting groove becomes complex. A part of the photo sensing plane of the photodiode is used for mounting the photodiode on the silicon substrate, so that the reduction of the sensitivity of the photodiode is inevitable.
For solving the problems in the above-mentioned conventional mounting structure, a carrier for optical semiconductor device having a slant face with the same angle as that of the slant face on the silicon substrate can be used in a manner so that the slant faces are parallel and directly contact with each other. By such a configuration, the photo sensing plane of the photodiode mounted on the carrier can be perpendicular to the optical axis of the optical fiber.
Positioning of the photodiode in a z-direction parallel to the optical axis of the optical fiber and a y-direction parallel to the height of the optical module can be adjusted by sliding the slant faces. Positioning of the photodiode in an x-direction perpendicular to the y-direction and the z-direction, however, depends on the shape of the silicon substrate formed by dicing, so that the photodiode cannot be positioned in the x-direction precisely. Thus, this mounting structure can be used for mounting the photodiode having relatively large tolerance, but it is considered not to satisfy the positioning accuracy of a data transmission module or a high frequency module using a surface emitting optical semiconductor device.
An object of the present invention is to provide a carrier for optical semiconductor device suitable for mounting, especially a surface emitting or sensing semiconductor device, having high productivity, downsized and splendid high frequency characteristic. Another object of the present invention is to provide a mounting structure of the optical semiconductor device and the optical module using the same.
A carrier for optical semiconductor device in accordance with the present invention has a device mounting face on which at least one optical semiconductor device is to be mounted and at least one positioning face slanted by a predetermined angle with respect to the device mounting face and disposed below a position at which the optical semiconductor device is mounted.
A mounting structure of the optical semiconductor device in accordance with the present invention comprises a carrier having the above-mentioned configuration and a substrate having at least one positioning face slanted by a predetermined angle with respect to a top face thereof.
An optical module in accordance with the present invention comprises a carrier and a substrate respectively having the above-mentioned configurations and at least one optical semiconductor device mounted on the device mounting face of the carrier and an optical fiber mounted on the top face of the substrate so as to be optically coupled with the optical semiconductor device.
When the carrier with the optical semiconductor device is fixed on the substrate, the positioning face of the carrier is contacted with the positioning face of the carrier, so that the device mounting face of the carrier has a predetermined angle, such as 90 degrees with respect to the top face of the substrate. Thus, the optical semiconductor device mounted on the carrier can be optically coupled with the optical fiber mounted on the substrate.